Women who have lived near busy roads and who have been exposed to exhaust fumes for decades tend to have decreased cognitive performance in old age, according to a startling report by a team of German scientists.

The closer the women lived to the highways, the higher was their exposure to particulate pollution and the more likely they were to show signs of mild memory and cognitive decline, the researchers wrote in a study published in the journal Environmental Research.

This is the first study to find an association between cognitive impairment and long-term exposure to air pollution due to traffic. It is also one of a handful of recent studies to report a link between air pollution and brain function in people.

The study was conducted by scientists at the Institute for Environmental Medicine Research (IUF) at Heinrich Heine University in Dusseldorf, Germany, and it examined 399 elderly women aged 68—79 who had lived at the same residential address in Germany for at least 20 years.

The State Environmental Agency had collected and monitored pollution levels for more than 25 years in the participants’ residential area. For the study, researchers estimated the particulate matter (PM) exposure of each subject using the measurements of the closest monitoring station.

Burning fuel - especially diesel fuel - in cars and other vehicles produces fine particles called particulate matter. Exposure to high levels of PM are associated with adverse cardiovascular and respiratory health effects. Based on animal studies, PM can also move directly from the lungs to the nervous system where it can cause inflammation and brain damage.

Brain and nerve inflammation are factors known to spur development of some types of degenerative diseases, such as Alzheimer’s Disease and Parkinson’s. So the researchers looked at whether chronic exposure to PM may be associated with the development of neurodegenerative diseases, which are increasing as the population ages.

A number of neurophysiological and cognitive tests including verbal recognition, learning capabilities, memory and recall were used to score individual cognitive performance.

Elderly women who lived closer to higher traffic areas had significantly lowered performance, called mild cognitive impairment (MCI) - a condition that is known to lead to Alzheimer’s Disease.

The MCI worsened as the residential distance to the busy streets got shorter. The link was seen in people living at most 50 metres from roads carrying up to 10,000 cars per day.

Even though other risk factors were controlled for, this human study has some limitations, say the authors. Living closer to a busy road also means exposure to higher levels of noise, which could be a contributing factor to the impaired cognition observed in the women who participated in the study.